1. Field of the Invention
The invention relates to an insulated wire and a coil using the same. In more detail, the invention relates to an insulated wire excellent in partial discharge resistance and high temperature processability, and a coil using the same.
2. Description of the Related Art
An insulated wire having an insulation layer (an insulating film) excellent in mechanical characteristics, heat resistance and solvent resistance has been proposed in which a polyimide synthesized from, e.g., pyromellitic dianhydride (PMDA) and 4,4′-diaminodiphenyl ether (ODA) is used for the insulation layer (see, e.g., JP-A 9-106712).
In recent years, industrial motors have been reduced in size and weight. In addition, inverter drive for improving dynamic performance, together with high voltage drive for high power output, is being developed rapidly.
Since the motor is driven at high voltage and at the same time is inverter-driven, the overlapping of the high voltage drive with the inverter drive increases the risk of partial discharge occurrence in an insulated wire of the motor. In an insulated wire with a low partial discharge inception voltage (PDIV), partial discharge is likely to occur at lower voltage and an insulation layer is gradually eroded due to the partial discharge occurred therein, which eventually causes insulation failure.
The PDIV of the insulated wire can be improved by increasing a film thickness of the insulation layer and decreasing the relative permittivity of the insulation layer. For high-power motors, a PDIV at a film thickness of 40 μm is needed to be not less than 900 Vp.
In case that the above-mentioned insulated wire having an insulation layer formed of a polyimide is used in such a high-power motor, the above-mentioned PDIV level may not be satisfied since the polyimide has relatively high relative permittivity and it is therefore necessary to increase a film thickness to manage to improve the PDIV. However, use of a thick insulation layer decreases a space factor of a conductor in a motor and this makes the motor difficult to output high power.